Conventionally, a water absorbing material made of a hydrophilic fiber such as pulp, and a water absorbing agent has been in widespread use in sanitary materials such as a disposable diaper, a sanitary napkin, and an incontinence pad, for the purpose of absorbing body fluids.
For enhancement in convenience, it has been demanded to make these sanitary materials thinner in recent years. This results in that, in a water absorbing material, a ratio of the hydrophilic fiber having low bulk specific gravity is decreased, and a ratio of the water absorbing agent having a high water absorbing rate and high bulk specific gravity is increased. Water absorbing agent usage in the water absorbing material is thus increased so as to make the sanitary material thinner without a reduction in its properties such as a water absorption amount.
Such a sanitary material having a small ratio of the hydrophilic fiber and a large ratio of the water absorbing agent is preferable for merely keeping liquid. However, when actually used in a disposable diaper, for example, such a sanitary material has a problem in distribution and diffusion of the liquid. For example, a large amount of the water absorbing agent turns into a soft gel state when absorbing water, and causes a gel blocking phenomenon in which water is prevented from being absorbed deeper into the water absorbing agent. This dramatically decreases a diffusing property of the liquid in the sanitary material. The ratio of the hydrophilic fiber to the water absorbing agent is inevitably limited so as to both avoid such a problem and maintain absorption characteristics of the water absorbing material. As a result, the sanitary material cannot be thinner than a certain limit.
In order to both suppress the gel blocking and realize a sufficient absorption amount, it is necessary to obtain a water absorbing agent that is excellent in a balance between an absorption capacity represented by a centrifugal retention capacity (CRC), for example, and liquid permeability represented by a saline flow conductivity (SFC), for example. However, they have such a relationship that an increase in one results in a decrease in the other. This makes it difficult to improve the relationship (balance) between them to a successful level. As means for attaining such an object, the following techniques have been known, for example.
Patent Document 1 discloses a water absorbent resin that has been processed with a tri- or more-valent cation. Patent Document 2 discloses a technique with which an electrostatic or stereoscopic spacer is used with a water absorbent resin. Patent Document 3 discloses a particulate water absorbing agent containing: water absorbent resin particles produced by (i) cross-linking a monomer containing acrylic acid and/or salt thereof, and (ii) further cross-linking a surface of each of particles thus obtained, which particles have been pulverized into irregular shapes; and an agent for enhancing liquid permeability. Thereby, Patent Document 3 provides a water absorbing agent having properties of both capillary suction pressure and liquid permeability.
Meanwhile, other than Patent Documents 1 through 3 whose object is to improve the balance between the absorption capacity and the liquid permeability, there has been proposed to mix various additives for the purpose of an improvement in various properties of a water absorbent resin.
For example, Patent Document 4 proposes a water absorbing agent composition made from a water absorbent resin, and an anionic surfactant having a carboxyl group or salt thereof. Thereby, Patent Document 4 provides a salt-tolerant water absorbing agent composition having excellent absorption performance with respect to a saline solution.
Further, Patent Document 5 proposes a particulate water absorbing agent containing: a particulate water absorbent resin having a surface-cross-linking structure; and polyvalent metal salt of organic acid having 7 or more carbon atoms in its molecule. Thereby, Patent Document 5 provides a particulate water absorbing agent having stable and excellent absorption performance, the particulate water absorbing agent: being advantageous in powder flowability when absorbing moisture, and under a dry condition where a moisture content is 0 mass % to 20 mass %; being excellent in handling during transportation etc.; and, being so tolerant to a mechanical impact in producing and carrying the particulate water absorbing agent, and manufacturing a water absorbing good with the water absorbing agent. The particulate water absorbing agent so tolerant to such a mechanical impact that the mechanical impact hardly reduces (i) absorption performance, and (ii) flowability when absorbing moisture.
Furthermore, Patent Document 6 discloses a high water-absorption resin composition produced by compounding high water-absorption resin powder of 100 pts.wt. with stearic acid of 0.1 pts.wt. to 5 pts.wt and inorganic powder of 0.1 pts.wt to 1 pts.wt. Thereby, Patent Document 6 provides a high water-absorption resin composition that not only improves absorption characteristics (a fish eye formation avoidability, dispersibility, an absorption speed, and absorption performance) and powder flowability but also has a dust formation avoidability.
Moreover, Patent Document 7 proposes a method of producing a water absorbing agent, including the step of mixing: water absorbent resin particles having an internal cross-linking structure that is obtained by polymerizing a water-soluble unsaturated monomer; organic acid (salt) having a low carbon number; and water-soluble polyvalent metal salt. Thereby, Patent Document 7 provides a method of producing a water absorbing agent by mixing water absorbent resin particles and a metal compound, wherein: it is possible to (i) suppress penetration of metal composition into the water absorbent resin particles, and (ii) realize high liquid permeability and uniform properties.
Further, Patent Document 8 proposes a method of producing a water absorbing agent, including the step of: mixing water absorbent resin particles and a cationic polymer compound in which a specific region is crosslinked. Thereby, Patent Document 8 provides a water absorbent resin that is excellent in an absorbency against pressure (AAP), a gel layer's flow rate under pressure (FRUP), and a saline flow conductivity (SFC), and can keep these effects for a long period.    [Patent Document 1]    Pamphlet of International Publication WO2001/74913 (published on Oct. 11, 2001)    [Patent Document 2]]]    Specification of US Patent Application Publication No. 2002/0128618 (published on Sep. 12, 2002    [Patent Document 3]    Pamphlet of International Publication WO2004/069915 (published on Aug. 19, 2004)    [Patent Document 4]    Japanese Unexamined Patent Publication No. Tokukaihei 6-345980 (published on Dec. 20, 1994)    [Patent Document 5]    Japanese Unexamined Patent Publication Tokukai 2004-261796 (published on Sep. 24, 2004]    [Patent Document 6)    Japanese Unexamined Patent Publication Tokukaisho 63-105064 (published on May 10, 1988)    [Patent Document 7]    Japanese Unexamined Patent Publication Tokukai 2005-344103 (published on Dec. 15, 2005)    [Patent Document 8]    Japanese Unexamined Patent Publication Tokukai 2003-62460 (published on Mar. 4, 2003)